Teeth play a critically important role in our lives. Loss of function reduces the ability to eat a balanced diet which results in negative consequences for systemic health. Loss of aesthetics can negatively impact social function. Both function and aesthetics can be restored with dental crowns and bridges. Ceramics are attractive dental restoration materials because of their aesthetics, inertness, and biocompatibility. However, ceramics are brittle and subject to premature failure, especially after repeated contact including slide-liftoff masticatory loading in a moist environment (Kim et al. (2007) Journal of Dental Research 86(11): 1046-1050; Lawn et al. (2001) The Journal of Prosthetic Dentistry 86(5): 495-510; Lawn et al. (2001) J Prosthet Dent 86(5): 495-510; Zhang et al. (in press) “Fatigue Damage in Ceramic Coatings from Cyclic Contact Loading with Tangential Component.” Journal of the American Ceramic Society) Fracture rates of ceramic restorations may seem low at 3-4% per year (Fradeani et al. (1997) Int. J. Prothodont. 10: 241-7; Malament et al. (1999) J. Prosthet. Dent. 81: 23-32; Sjogren et al. (1999) Int. J. Prosthodont. 12: 122-8; Sailer et al. (2006) Quintessence International 37(9): 685-693; Sailer et al. (2007) Clin. Oral Impl. Res. 18(3): 86-96; Pjetursson et al. (2007) Clin. Oral Impl. Res. 18(3): 73-85). However, failure can cause significant patient discomfort and loss of productive lifestyle. The vulnerability of dental ceramic restorations is exacerbated by damage, fatigue loading, and moisture.
According to a survey conducted by American Dental Association, more than 45 million new dental crowns, of which over 37 million were porcelain (ceramic) based, were provided by dentists in 1999 (ADA (2002). “The 1999 Survey of Dental Services Rendered.”). As the population ages, the number will increase. Despite continuous efforts to improve the strength of dental ceramics, all-ceramic dental crowns continue to fail at a rate of approximately 3-4% each year (Burke et al. (2002) J. Adhes Dent 4(1): 7-22). The highest fracture rates are on posterior crowns and bridges where stresses are greatest. Dental crowns generate over $2 billion each year in revenues with 20% of the units being all-ceramic (Nobel Biocare 2004). Dental ceramics that best mimic the optical properties of enamel and dentin are predominantly glassy materials principally feldspar (a group of minerals having main constituents of silica and alumina) (Kelly (1997) Annual Reviews of Materials Science 27: 443-68; Kelly (2004) Dent. Clin. N. Am. 48: 513-30). The original dental porcelain contained high feldspathic glass content and was extremely brittle and weak (S(strength) approximately ˜60 PMa) (McLean, J. W. (1979) The Science and Art of Dental Ceramics. Chicago, Quintessence Publishing Co. Inc.; Binns, D. (1983) The Chemical and Physical Properties of Dental Porcelain. Chicago, Quintessence Publishing Co. Inc.). Therefore, despite the aesthetic advantage, the early porcelain crowns were not widely used in dentistry (Van, N. R. (2002). “An Introduction to Dental Materials.” 231-46).
Dental ceramics have become increasingly popular as restorative materials due to improvements in strength. Several methods have been developed to improve the strength of dental ceramics including adding uniformly disperse appropriate filler particles throughout a glass matrix, referred to as “dispersion strengthening” (McLean et al. (1965) Br. Dent. J. 119: 251-67). The first fillers used in dental ceramics were leucite particles (Denry (1996) Crit. Rev. Oral. Biol. Med. 7: 134-43). Commercial dental ceramics containing leucite as a dispersion strengthening fillers include IPS Empress (S˜120 PMa) (Ivoclar-Vivadent, Schaan, Liechtenstein) and Finesse All-ceramic (S approximately 125 MPa) (Dentsply Prosthetics, York, Pa.). Particle strengthening can also be achieved by heat-treating the glass to facilitate the precipitation and subsequent growth of crystallites within the glass, termed “ceraming”. Dental ceramics produced using the ceraming process are called glass-ceramics. Several commercial products such as Dicor (S˜160 MPa) (Dentsply), IPS Empress II (S˜350 MPa) (Ivoclar-Vivadent) and, more recently, IPS e.max Press (S˜525 MPa) (Ivoclar-Vivadent) are examples. The leucite-strengthened porcelains and the glass-ceramics are translucent, so single layer (monolithic) crowns can be made from these materials. However, only moderate strength increases can be achieved via the particle strengthening techniques. Therefore, monolithic ceramic crowns experience high failure rates range from 4-6% for Dicor molar crowns (Malament et al. (1999) J. Prosthet. Dent. 81: 23-32) and 3-4% per year for IPS Empress crowns (Fradeani et al. (1997) Int. J. Prothodont. 10: 241-7; Sjogren et al. (1999). Int. J. Prosthodont. 12: 122-8). Note: comprehensive clinical reports on the new IPS e.max Press crowns are not available at this stage.
The current approach to the fracture problem of monolithic crowns is a layer-structure with aesthetic but weak porcelain veneers fused onto strong but opaque ceramic cores. This involves an increase in crystalline content (from approximately 40 vol % to 99.9 vol %) accompanied by a reduction in glass content. The first successful strengthened core ceramic was made of feldspathic glass filled with approximately 40 vol % alumina particles (McLean et al. (1965). Br. Dent. J. 119: 251-67). The alumina fillers increased the flexural strength of the ceramic to approximately 120 MPa with a trade off in translucency; hence veneering was required. Using McLean's approach, in 1983 Coors Biomedical (Golden, Colo.) developed Cerestore all-ceramic crowns with a ceramic core containing ˜60 vol % of alumina (Sozio et al. (1983). J. Prosthet. Dent. 69: 1982-5). However, following problems with fractured crowns the manufacturer withdrew the system. A similar product from the same era, the Hi-Ceram crown (Vita, Bad Säckingen, Germany) with its core material containing about the same amount of alumina as the Cerestore core, also failed to meet the satisfactory for posterior restorations (Bieniek et al. (1994). Schweitz Monatsschr Zahnmed 104: 284-9). The Hi-Ceram crown was replaced by In-ceram crown (Vita) in 1990. The In-ceram crown had a core that was fabricated by lightly sintering an alumina powder compact and then infiltrating the still porous alumina matrix with a low viscosity glass. The final core material contained approximately 70 vol % of alumina and had a flexural strength of approximately 450 MPa (Probster (1992) Int J Prosthodont 5(5): 409-14). In 1993, Procera (Nobel Biocare, Göteborg, Sweden) presented the all-ceramic crown concept (Anderson et al. (1993). Acta Odontol Scand 51: 59-64), where the fully dense core material contained 99.9 vol % alumina and displayed a flexural strength of 675 MPa. Several years later, even stronger Y-TZP ceramic was introduced to dentistry as a core material with a flexural strength over 1200 MPa.
Although documentation regarding the clinical performance of the zirconia core backed crowns is still limited, laboratory in vitro tests (B. Kim et al, (2007) Journal of Dental Research, 86(2): 142-146) and anecdotal clinical reports (Donovan (2005) Journal of Esthetic and Restorative Dentistry 17(3): 141-3.) indicate that the zirconia cores are very fracture resistant. However, a frequent problem is fracture of the porcelain veneer. Despite significant improvements in the performance of existing dental ceramics, the structural stability of all-ceramic systems remains less reliable than metal-ceramic systems (porcelain veneers fused onto metal copings) (Kelly (2004) Dent. Clin. N. Am. 48: 513-30). While efforts in improving the structural performance of all-ceramic crowns have been focused on making monolithic materials stronger or fabricating stronger cores to support weak, but aesthetic porcelain veneers, few innovative approaches have emerged to develop more damage resistant and longer lasting ceramic crowns. This is due in part to the lack of current knowledge of damage modes that could occur in a ceramic crown under mastication.
Unfortunately, no current materials, including monolithic ceramics stronger (orthopedic and dental prostheses) or strong cores to support weak, but aesthetic porcelain veneers (dental prostheses) can effectively suppress both contact and flexural damages. In addition, veneered strong ceramic dental prostheses have a dense, high purity crystalline structure at the cementation internal surface that cannot be readily adhesively bonded to tooth dentin as support. Surface roughening treatment such as particle abrasion is commonly used to enhance the ceramic-luting agent bond. However, particle abrasion also introduces surface flaws or microcracks that can cause deterioration in the long-term flexural strength of ceramic prostheses. (Zhang et al. (2004) Journal of Biomedical materials research 71B(2): 381-6; Zhang et al. (2005) Journal of Biomedical materials research 72B: 388-92; Zhang et al. (2006) The International Journal of Prosthodontics 19(5): 442-8).
Recent advances in theoretical and experimental work have shown that functionally graded materials, referred to as FGMs, may provide unprecedented resistance to contact damage (Suresh et al. (2003) U.S. Pat. No. 6,641,893; Suresh et al. (1997) Acta Materialia 45(4): 1307-21; Jitcharoen et al. (1998) Journal of the American Ceramic Society 81(9): 2301-8; Suresh et al. (1999) Acta Materialia 47(14): 3915-3926). Such damage resistance cannot be realized with conventional homogeneous materials. FGMs are made of two materials that are combined so that the surface of the FGM is composed entirely of material A, and the interior is composed entirely of material B. Additionally, there is a continuous change in the relative proportions of the two materials from the surface to interior. One known FGM is a thick ceramic block, alumina or silicon nitride, infiltrated with a low elastic modulus aluminosilicate glass or oxynitride glass (SiAlYON), respectively, on one surface to produce a graded glass/ceramic (G/C) structure that suppresses contact damage at the top, occlusal surface (Jitcharoen et al. (1998) Journal of the American Ceramic Society 81(9): 2301-8). However, upon infiltration of dense ceramics, the glass penetrates the grain boundaries and grain boundary triple junctions, and as a result, the ceramic grains gradually separate. This leads to an increase in volume at the surface of graded structure and is accompanied by warpage or bending of the specimens where the glass-impregnated surface is convex.
Zirconium dioxide (ZrO2), sometimes known as zirconia, is a white crystalline oxide of zirconium. Its most naturally occurring form, with a monoclinic crystalline structure, is the rare mineral, baddeleyite. Pure ZrO2 has a monoclinic crystal structure at room temperature and transitions to tetragonal and cubic at increasing temperatures. The volume expansion caused by the cubic to tetragonal to monoclinic transformation induces very large stresses, and will cause pure ZrO2 to crack upon cooling from high temperatures. Several different oxides are added to zirconia to stabilize the tetragonal and/or cubic phases: magnesium oxide (MgO), yttrium oxide, (Y2O3), calcium oxide (CaO), and cerium oxide (Ce2O3), amongst others.
If sufficient quantities of the metastable tetragonal phase zirconia is present, then an applied stress, magnified by the stress concentration at a crack tip, can cause the tetragonal phase to convert to monoclinic, with the associated volume expansion. This phase transformation can then put the crack into compression, retarding its growth, and enhancing the fracture toughness. This mechanism is known as transformation toughening, and significantly extends the reliability and lifetime of products made with partially stabilized zirconia. A special case of zirconia is that of tetragonal zirconia polycrystaline or TZP, which is indicative of polycrystalline zirconia composed of only the metastable tetragonal phase. This material is also used in the manufacture of frameworks for the construction of dental restorations such as crowns and bridges which are then veneered with a dental feldspathic porcelain, as well as femoral heads for the total hip replacement.